Pixel structure of a thin film transistor liquid crystal display and fabricating method thereof

ABSTRACT

A method of fabricating a pixel structure of a thin film transistor liquid crystal display is provided. A transparent conductive layer and a first metallic layer are sequentially formed over a substrate. The first metallic layer and the transparent conductive layer are patterned to form a gate pattern and a pixel electrode pattern. A gate insulating layer and a semiconductor layer are sequentially formed over the substrate. A patterning process is performed to remove the first metallic layer in the pixel electrode pattern while remaining the gate insulating layer and the semiconductor layer over the gate pattern. A second metallic layer is formed over the substrate. The second metallic layer is patterned to form a source/drain pattern over the semiconductor layer. A passivation layer is formed over the substrate and then the passivation layer is patterned to expose the transparent conductive layer in the pixel electrode pattern.

CROSS-REFERENCE TO RELATED APPLICATION

This is a divisional application of and claims the priority benefit ofpatent application Ser. No. 11/180,090, filed on Jul. 11, 2005, whichclaims the priority benefit of Taiwan application serial no. 93124164,filed on Aug. 12, 2004. The entirety of each of the above-mentionedpatent applications is incorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a pixel structure of a thin filmtransistor liquid crystal display and a fabricating method thereof. Moreparticularly, the present invention relates to a pixel structure of athin film transistor liquid crystal display fabricated in four maskprocesses.

2. Description of the Related Art

A thin film transistor liquid crystal display mainly comprises a thinfilm transistor array substrate, a color filter array substrate and aliquid crystal layer sandwiched between the two substrates. The thinfilm transistor array substrate has a plurality of pixel structures, andeach pixel structure comprises a thin film transistor and a pixelelectrode.

The pixel structure of most of the thin film transistor liquid crystaldisplay is fabricated in five mask processes. The first mask process isused to pattern a first metallic layer so as to form a scanning line anda gate electrode of the thin film transistor. The second mask process isused to define a channel layer and an ohmic contact layer of the thinfilm transistor. The third mask process is used to pattern a secondmetallic layer to form a data line and a pair of source/drain electrodesof the thin film transistor. The fourth mask process is used to patterna passivation layer and the fifth mask process is used to pattern atransparent conductive layer to form a pixel electrode.

However, as the size of thin film transistor liquid crystal displaypanel continues to increase, several problems such as a drop in theyield and throughput have to be overcome. Therefore, if the number ofmask processes for forming the thin film transistor is reduced, thatmeans the number of exposure process in fabricating the thin filmtransistor is reduced, the manufacturing time and cost can be reduced.

At present, methods of fabricating thin film transistor liquid crystaldisplay using four mask processes have been developed. However, thesemethods often utilize a halftone pattern design on a photo-mask toachieve the reduction of one mask process. However, using the halftonepattern on a photo-mask to form the display also exists some problems.For example, it is more difficult to design the pattern layout on thephoto-mask and the photoresist selectivity should be consideredseriously. Furthermore, using the photo-mask having a halftone patternthereon to perform an exposure process usually causes the patternedphotoresist to have non-uniform patterns.

SUMMARY OF THE INVENTION

Accordingly, at least one objective of the present invention is toprovide a method of fabricating a pixel structure of a thin filmtransistor liquid crystal display with four mask processes in which nohalftone technique is used in photo-masks.

At least a second objective of the present invention is to provide apixel structure of a thin film transistor liquid crystal display thatcan be fabricated with four mask processes.

To achieve these and other advantages and in accordance with the purposeof the invention, as embodied and broadly described herein, theinvention provides a method of fabricating a pixel structure of a thinfilm transistor liquid crystal display. A transparent conductive layerand a first metallic layer are sequentially formed over a substrate. Afirst mask process is performed to pattern the first metallic layer andthe transparent conductive layer so as to form a gate pattern and apixel electrode pattern. A gate insulating layer and a semiconductorlayer are sequentially formed over the substrate to cover the gatepattern and the pixel electrode pattern. A second mask process isperformed to remove the first metallic layer in the pixel electrodepattern but the gate insulating layer and the semiconductor layer overthe gate pattern are remained. Thereafter, a second metallic layer isformed over the substrate. A third mask process is performed to patternthe second metallic layer so as to form a source pattern and a drainpattern over the remained semiconductor layer. After that, a passivationlayer is formed over the substrate. A fourth mask process is performedto pattern the passivation layer so that the transparent conductivelayer in the pixel electrode pattern is exposed.

The present invention also provides a pixel structure of a thin filmtransistor liquid crystal display comprising a thin film transistor, apixel electrode pattern and a passivation layer. The thin filmtransistor is disposed on a surface of a substrate. The thin filmtransistor comprises a gate pattern, a gate insulating layer disposed onthe gate pattern and the substrate, a semiconductor layer covering thegate insulating layer and a source pattern and a drain pattern formedover the semiconductor layer. The pixel electrode pattern is disposed onthe surface of the substrate and is electrically connected to the drainpattern of the thin film transistor. The passivation layer covers thethin film transistor but exposes the pixel electrode pattern.

In the present invention, only four mask processes are required tofabricate the pixel structure. Comparing with the five mask processes inthe conventional method, the number of mask processes can be reduced.Therefore, the throughput is increased and the manufacturing cost downcan be achieved.

Furthermore, halftone patterns are not used on the photo-mask in thepresent invention and hence problems related to the layout design of thephoto-mask and photoresist selectivity can be avoided. Moreover, theproblem of non-uniform pattern of the patterned photoresist after theexposure process using the photo-mask having halftone patterns thereondoes not exist.

It is to be understood that both the foregoing general description andthe following detailed description are exemplary, and are intended toprovide further explanation of the invention as claimed.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings are included to provide a furtherunderstanding of the invention, and are incorporated in and constitute apart of this specification. The drawings illustrate embodiments of theinvention and, together with the description, serve to explain theprinciples of the invention.

FIG. 1 is a top view showing a pixel structure of a thin film transistorliquid crystal display according to one preferred embodiment of thepresent invention.

FIGS. 2A through 2H are schematic cross-sectional views showing thesteps for fabricating a pixel structure of a thin film transistor liquidcrystal display according to one preferred embodiment of the presentinvention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Reference will now be made in detail to the present preferredembodiments of the invention, examples of which are illustrated in theaccompanying drawings. Wherever possible, the same reference numbers areused in the drawings and the description to refer to the same or likeparts.

The pixel structure of a thin film transistor liquid crystal display inthe present invention is fabricated with four mask processes in whichthe halftone technique on photo-masks is not required. The substratehaving a plurality of pixel structures thereon and fabricated using themethod of the present invention can be combined with a color filtersubstrate and a liquid crystal layer in many ways to form a thin filmtransistor liquid crystal display panel. The following is a descriptionof the preferred embodiment of the present invention but should not beused to limit the scope of the present invention.

FIG. 1 is a top view showing a pixel structure of a thin film transistorliquid crystal display according to one preferred embodiment of thepresent invention. FIGS. 2A through 2H are schematic cross-sectionalviews showing the steps for fabricating a pixel structure of a thin filmtransistor liquid crystal display according to one preferred embodimentof the present invention. As shown in FIGS. 1 and 2A, a transparentconductive layer 102 and a first metallic layer 104 are sequentiallyformed over a substrate 100. In one preferred embodiment, the substrate100 further comprises an area for forming thin film transistors (T), anarea for forming pixel electrodes (P), an area for forming storagecapacitors (C) and an area for forming bonding pads B and B′. Thesubstrate 100 can be a transparent glass substrate or a transparentplastic substrate, for example. The transparent conductive layer 102 canbe a metal oxide layer including, for example, an indium-tin-oxide (ITO)layer, an indium-zinc-oxide (IZO) layer or the like. The first metalliclayer 104 can be fabricated from chromium (Cr), tungsten (W), tantalum(Ta), titanium (Ti), molybdenum (Mo), aluminum (Al) layer or an alloythereof, for example. In particular, if the transparent conductive layer102 is fabricated from ITO or IZO material, the first metallic layer 104is preferably selected from chromium (Cr), tungsten (W), tantalum (Ta),titanium (Ti), molybdenum (Mo) or an alloy thereof.

A first mask process is performed to form a patterned photoresist layer106 over the first metallic layer 104. Thereafter, using the photoresistlayer 106 as an etching mask, an etching process is carried out topattern the first metallic layer 104 and the transparent conductivelayer 102 so as to form a patterned first metallic layer 104 a and apatterned transparent conductive layer 102 a as shown in FIG. 2B. In onepreferred embodiment, the first mask process defines a gate pattern 108in the area for forming thin film transistors (T), a pixel electrodepattern 110 in the area for forming pixel electrodes (P) and a scanningline 150 (as shown in FIG. 1) electrically connecting with the gatepattern 108.

In another preferred embodiment, the first mask process further definesa lower electrode pattern 112 in the area for forming storage capacitors(C). The storage capacitor is a capacitor-on-gate (Cs on gate), forexample. In another preferred embodiment, the first mask process furtherdefines a bonding pad pattern 114 electrically connecting with thescanning line 150 in the area for forming bonding pads (B) near one ofthe edges of the substrate 100. Similarly, the first mask process alsodefines an independent bonding pad pattern 114 a (having a cross sectionsimilar or identical to the bonding pad B) in the area for formingbonding pads B′ near another edge of the substrate 100. In anotherpreferred embodiment, the first mask process also defines a lowerelectrode pattern 112 and a bonding pad pattern 114.

As shown in FIGS. 1 and 2C, a gate insulating layer 116 and asemiconductor layer 118 are sequentially formed over the substrate 100to cover the aforementioned structures. In one preferred embodiment, thegate insulating layer 116 is fabricated from silicon nitride, siliconoxide or silicon oxynitride, for example. The semiconductor layer 118 isconstituted of a channel material layer (for example, an amorphoussilicon layer) and an ohmic contact material layer (for example, a dopedamorphous silicon layer).

A second mask process is performed to form a patterned photoresist layer120 over the semiconductor layer 118. Using the photoresist layer 120 asan etching mask, an etching process is carried out to pattern thesemiconductor layer 118 and the gate insulating layer 116 so as to forma patterned semiconductor layer 118 a and a gate insulating layer 116 aas shown in FIG. 2D. In the meantime, the first metallic layer 104 a inthe pixel electrode pattern 110 is removed while the transparentconductive layer 102 a in the pixel electrode pattern 110 is remained.In one preferred embodiment, the semiconductor layer 118 a and the gateinsulating layer 116 a over the gate pattern 108 are also remained inthe second mask process.

In another preferred embodiment, the second mask process furthercomprises remaining the semiconductor layer 118 a and the gateinsulating layer 116 a over the electrode pattern 112, which serve as acapacitor dielectric layer. In another preferred embodiment, the secondmask process further comprises removing a portion of the semiconductorlayer 118 a and the gate insulating layer 116 a over the bonding padpattern 114, 114 a. In addition, a portion of the first metallic layer104 a of the bonding pad pattern 114, 114 a is removed so that thetransparent conductive layer 102 a of the bonding pad pattern 114, 114 ais exposed. In another preferred embodiment, the second mask processfurther comprises remaining the semiconductor layer 118 a and the gateinsulating layer 116 a over the lower electrode pattern 112 and thebonding pad pattern 114, 114 a and removing a portion of the firstmetallic layer 104 a of the bonding pad pattern 114, 114 a so that thetransparent conductive layer 102 a of the bonding pad pattern 114, 114 ais exposed.

As shown in FIGS. 1 and 2E, a second metallic layer 122 is formed overthe substrate 100 to cover the aforementioned structures. In onepreferred embodiment, the second metallic layer is fabricated fromchromium (Cr), tungsten (W), tantalum (Ta), titanium (Ti), molybdenum(Mo), aluminum (Al) or an alloy thereof.

A third mask process is performed to form a patterned photoresist layer124 over the second metallic layer 122. Using the photoresist layer 124as an etching mask, an etching process is carried out to pattern thesecond metallic layer 122 so as to form a patterned second metalliclayer 122 a as shown in FIG. 2F. In one preferred embodiment, the secondmetallic layer 122 a over the gate pattern 108 includes a source pattern126 and a drain pattern 128. The drain pattern 128 is electricallyconnected to the pixel electrode pattern 110. Furthermore, the thirdmask process further comprises defining a data line 160 electricallyconnecting to the source pattern 126 (as shown in FIG. 1). In onepreferred embodiment, a definite thickness of the semiconductor layer118 a between the source pattern 126 and the drain pattern 128 isremoved to form a semiconductor layer 118 b when the second metalliclayer 122 is patterned. Thus, a channel 119 is formed between the sourcepattern 126/drain pattern 128 and the gate pattern 108.

In another embodiment, the third mask process further comprisesremaining the second metallic layer 122 a over the lower electrodepattern 112 serving as an upper electrode 129 of a pixel storagecapacitor. Furthermore, the upper electrode 129 is electricallyconnected to the pixel electrode pattern 110. Hence, the upper electrode129, the lower electrode pattern 112 and the dielectric material layerbetween the two electrodes (the gate insulating layer 116 a and thesemiconductor layer 118 a) together form a pixel storage capacitor. Inanother preferred embodiment, the third mask process further comprisesremaining the second metallic layer 122 a over the bonding pad pattern114. Furthermore, the second metallic layer 122 a is electricallyconnected to the first metallic layer 104 a and the transparentconductive layer 102 a of the bonding pad pattern 114. In addition, asecond metallic layer 122 a electrical connecting with the data line 160is formed in the area for forming the bonding pads B′ near to one edgeof the substrate 100 to be a portion of the bonding pad pattern 114 a.Preferably, the bonding pad B′ structure and the bonding pad B structureare identical or similar. In another preferred embodiment, the thirdmask process further comprises remaining the second metallic layer 122 aover the lower electrode pattern 112 and the bonding pad pattern 114,114 a.

As shown in FIGS. 1 and 2G, a passivation layer 130 is formed over thesubstrate 100 to cover the aforementioned structures. In one preferredembodiment, the passivation layer 130 is fabricated from a materialincluding silicon oxide, silicon nitride, silicon oxynitride or anorganic compound, for example.

Thereafter, a fourth mask process is performed to form a patternedphotoresist layer 132 over the passivation layer 130. Using thephotoresist layer 132 as an etching mask, an etching process is carriedout to pattern the passivation layer 130 so as to form a patternedpassivation layer 130 a as shown in FIG. 2H. In one preferredembodiment, the patterned passivation layer 130 a exposes thetransparent conductive layer 102 a of the pixel electrode pattern 110.In another preferred embodiment, the patterned passivation layer 130 afurther exposes a portion of the transparent conductive layer 102 a ofthe bonding pad pattern 114, 114 a so that the bond pad pattern 114, 114a can connect electrically to an external circuit.

The aforementioned pixel structure formed in four mask steps comprises athin film transistor (T), a pixel electrode (P) and a passivation layer130 a. The thin film transistor (T) is disposed on a surface of asubstrate 100. The thin film transistor (T) comprises a gate pattern108, a gate insulating layer 116 a disposed on the gate pattern 108 andthe substrate 100, a semiconductor layer 118 a covering the gateinsulating layer 116 a and a source pattern/drain pattern 126/128 formedover the semiconductor layer 118 a. The pixel electrode pattern 110 ofthe pixel electrode (P) is disposed on the surface of the substrate 100and is electrically connected to the drain pattern 128 of the thin filmtransistor (T). In addition, the passivation layer 130 a covers the thinfilm transistor (T) but exposes the pixel electrode pattern 110.

In one preferred embodiment, the drain pattern 128 of the thin filmtransistor (T) covers a portion of the pixel electrode pattern 110. Inanother preferred embodiment, the gate pattern 108 comprises a lowertransparent conductive layer 102 a and an upper metallic layer 104 a. Inone preferred embodiment, the gate insulating layer 116 a in the thinfilm transistor (T) is formed between the semiconductor layer 118 a andthe gate pattern 108.

In one preferred embodiment, the pixel structure of the presentinvention further comprises a storage capacitor (C) disposed on thesubstrate 100. The storage capacitor (C) comprises a lower electrode112, an upper electrode 129 (the metallic layer 122 a) formed over thelower electrode 112 and a dielectric material layer (for example, thegate insulating layer 116 a and the semiconductor layer 118 a)sandwiched between the two electrodes. In one preferred embodiment, thelower electrode 112 comprises a lower transparent conductive layer 102 aand an upper metallic layer 104 a. In another preferred embodiment, theupper electrode 129 covers a portion of the pixel electrode pattern 110.

In one preferred embodiment, the pixel electrode of the presentinvention further comprises bonding pads B, B′ disposed close to the twoedges of the substrate 100. The bonding pad pattern 114 of the bondingpads B is electrically connected to the scanning line 150. The bondingpad pattern 114 comprises a lower transparent conductive layer 102 a andan upper metallic layer 104 a. Furthermore, the bonding pads B′ of thebonding pad pattern 114 a is electrically connected to the data line160. The bonding pads B′ have a structure similar to the bonding pads B.The passivation layer 130 a exposes the lower transparent conductivelayer 102 a of the bonding pad patterns 114, 114 a so that the bondingpad patterns 114, 114 a can connect electrically with an externalcircuit.

Another pixel structure of a thin film transistor liquid crystal displayand a fabricating method thereof according to a second embodiment arealso provided. The second embodiment is similar to the first embodimentof the present invention. The difference is that the first metalliclayer 104 formed over the transparent conductive layer 102 as shown inFIG. 2A has a multi-layered metallic structure. The multi-layeredmetallic structure is fabricated from two or three metallic layersselected from a chromium (Cr) layer, a tungsten (W) layer, a tantalum(Ta) layer, a titanium (Ti) layer, a molybdenum (Mo) layer, an aluminum(Al) layer and an alloyed layer thereof, for example. In one embodiment,the first metallic layer 104 is an Al/Cr/Al or Mo/Al/Mo three-layeredstructure, or a Cr/Al two-layered structure, for example. If thetransparent conductive layer 102 is fabricated from ITO or IZO material,the layer in the multi-layered metallic layer 104 be contact with thetransparent conductive layer 102 is preferably fabricated from chromium(Cr), tungsten (W), tantalum (Ta), titanium (Ti), molybdenum (Mo) or analloy thereof.

In FIG. 2B, the gate pattern 108 in the area for the thin filmtransistor (T), the pixel electrode pattern 110 in the area for thepixel electrode (P) and the metallic layer portion of the scanning line150 (shown in FIG. 1) are multi-layered metallic layers. Similarly, themetallic portion of the lower electrode pattern 112 and the bonding padpattern 114, 114 a may be a multi-layered metallic layer structure.

The steps performed in FIGS. 2C and 2D are similar to the firstembodiment and hence detailed description is omitted. In FIG. 2E, thesecond metallic layer 122 on the substrate 100 is a multi-layeredmetallic structure comprising two or three metallic layers selected froma group of materials including, for example, chromium (Cr), tungsten(W), tantalum (Ta), titanium (Ti), molybdenum (Mo), aluminum (Al) and analloy thereof. Here, the number of metallic layers and the constituentmaterial of each metallic layer of the second metallic layer 122 can beidentical or different from that of the first metallic layer 104.

Thus, in step 2F, the metallic portion of the source pattern 126, thedrain pattern 128 and the data line 160 has a multi-layered metallicstructure. Similarly, the metallic portion of the upper electrode 129 ofthe pixel storage capacitor is a multi-layered metallic layer structure.

Thereafter, the steps in FIGS. 2G and 2H are carried out. Since thesteps in FIGS. 2G and 2H are identical to the first embodiment, detaileddescription is omitted.

In the present invention, only four mask processes are required tofabricate the pixel structure. Comparing with the five mask processes inthe conventional method, the number of mask processes can be reduced.Therefore, the throughput is increased and the manufacturing cost downcan be achieved.

Furthermore, halftone patterns are not used on the photo-masks in thepresent invention. Hence, problems related to the layout design of thephoto-mask and the photoresist selectivity can be avoided. Moreover, theproblem of non-uniform pattern of the patterned photoresist after theexposure process using the photo-mask having halftone patterns thereondoes not exist.

It will be apparent to those skilled in the art that variousmodifications and variations can be made to the structure of the presentinvention without departing from the scope or spirit of the invention.In view of the foregoing, it is intended that the present inventioncover modifications and variations of this invention provided they fallwithin the scope of the following claims and their equivalents.

1. A pixel structure of a thin film transistor liquid crystal display,comprising: a thin film transistor disposed on a surface of a substrate,wherein the thin film transistor comprises a gate pattern, a gateinsulating layer disposed on the gate pattern and the substrate, asemiconductor layer covering the gate insulating layer and a sourcepattern and a drain pattern formed over the semiconductor layer; a pixelelectrode pattern disposed on the surface of the substrate, wherein thepixel electrode pattern is electrically connected to the drain patternof the thin film transistor; a passivation layer covering the thin filmtransistor but exposing the pixel electrode pattern; and a storagecapacitor, disposed on the substrate, comprising a lower electrode, anupper electrode and a capacitor dielectric layer, wherein the lowerelectrode comprises a lower transparent conductive layer and an uppermetallic layer, and the upper electrode covers a portion of the pixelelectrode pattern.
 2. The pixel structure of claim 1, wherein the drainpattern covers a portion of the pixel electrode pattern.
 3. The pixelstructure of claim 1, wherein the gate pattern comprises a lowertransparent conductive layer and an upper metallic layer.
 4. The pixelstructure of claim 3, wherein the upper metallic layer comprises asingle metallic layer, an alloy layer or a multi-layered metallic layer.5. The pixel structure of claim 1, wherein the gate insulating layer inthe thin film transistor is formed between the semiconductor layer andthe gate pattern.
 6. The pixel structure of claim 1, wherein the upperelectrode covers a portion of the pixel electrode pattern.
 7. The pixelstructure of claim 1, wherein the pixel structure further comprises abonding pad pattern disposed on two edges of the substrate.
 8. The pixelstructure of claim 7, wherein the bonding pad comprises a lowertransparent conductive layer and an upper metallic layer, and the uppermetallic layer exposes a portion of the lower transparent conductivelayer.
 9. A pixel structure of a thin film transistor liquid crystaldisplay, comprising: a thin film transistor disposed on a surface of asubstrate, wherein the thin film transistor comprises a gate pattern, agate insulating layer disposed on the gate pattern and the substrate, asemiconductor layer covering the gate insulating layer and a sourcepattern and a drain pattern formed over the semiconductor layer; a pixelelectrode pattern disposed on the surface of the substrate, wherein thepixel electrode pattern is electrically connected to the drain patternof the thin film transistor; a passivation layer covering the thin filmtransistor but exposing the pixel electrode pattern; a storagecapacitor, disposed on the substrate, comprising a lower electrode, anupper electrode and a capacitor dielectric layer, wherein the lowerelectrode comprises a lower transparent conductive layer and an uppermetallic layer; and a bonding pad pattern disposed on two edges of thesubstrate, wherein the bonding pad comprises a lower transparentconductive layer and an upper metallic layer, the upper metallic layerexposes a portion of the lower transparent conductive layer, and thepassivation layer exposes the lower transparent conductive layer of thebonding pad pattern.
 10. The pixel structure of claim 1, wherein thedrain pattern and the source pattern are single metallic layers, alloylayers or multi-layered metallic layers.